1. Technical Field
This invention relates in general to percent compositional analysis of a binary gas mixture, and more particularly, to a binary gas analyzer instrument and analysis method effective to indicate percent contamination of a cooling gas within a turbine generator.
2. Description of Prior Art
For many years, all large steam turbine driven electrical generators (turbo generators) have been designed to capitalize on the improved cooling capabilities of hydrogen, contrasted with air. Hydrogen gas has been found to be an excellent cooling medium, however, it is not without problems. Air is usually an ever present and potential contaminant, and due to the combustible nature of a hydrogen-oxygen mixture, precaution must be taken to prevent its existence at dangerous levels. High-purity hydrogen will not support combustion, and as long as the purity is above 95% there is no danger of explosion.
Conversely, if the percent air concentration in a hydrogen-air mixture exceeds 15% by volume, the mixture is potentially explosive. Thus, it is important that in operating hydrogen gas cooled generators, the relative composition of contaminant air in the hydrogen cooling gas be continually monitored.
The maximum risk of an explosive mixture developing occurs when the generator is out of service, particularly during time of purging or refilling, and when various types of maintenance or repair activities are underway. In order to lessen the possibility of an explosive mixture developing, it is standard practice in filling the generator for air to first be displaced from the generator by carbon dioxide (CO.sub.2), and then for CO.sub.2 to be displaced by filling with hydrogen (H.sub.2). Purging of the generator entails the reverse process.
The principle prior art means for monitoring air contamination within a gas cooled generator was developed by the assignee of the present invention several decades ago. This monitoring means consists of a Golay cell and a reference cell mounted in a brass housing together with a power unit, a flowrator, a wheatstone bridge circuit, and an indicating meter. Two arms of the bridge consist of a filament each and the other two consist of bridge completion resistors of comparable resistance value. One of the filaments is enclosed in the reference cavity of the detector in sealed air, while the other is placed in the measuring cell having the mixture to be measured. Imbalances in thermal conductivity between the two cells, as measured by imbalances in the resistances of the two filaments, are indicated by the bridge imbalance current and serve as the measure of the composition of the mixture. Although the instrument offers a reasonable indication of the composition of the binary mixtures used, it does have several shortcomings. For example, this prior art instrument is very sensitive to variations in supply voltage and to the temperature of the detector block. Further, for air and CO.sub.2 in hydrogen detection, the accuracy and sensitivity are relatively poor in the critical 0% to 20% contamination region, and the instrument requires manual calibration during each use.
Another prior art monitoring system is disclosed in U.S. Pat. No. 4,440,017, also owned by the same assignee as the present invention. This patent describes a dual, water and hydrogen gas cooling system for an electrical generator and a device for monitoring leakage of hydrogen gas into the water cooling system, but not gas contamination of the hydrogen gas.
During the years of prior art monitoring methods of hydrogen cooled turbo generators, there have been occassional cases in which internal explosions have occured. Therefore, there is a genuine need for an improved gas mixture monitoring system to assist an operator in safely performing the normal functions of filling, purging and operating gas cooled turbine driven generators.